Jake Simon reads a stressed tree the way my grandmother read weather in the Guatemalan highlands: in the body, before the instruments confirm it. Leaves curling at their edges weeks before they drop. A canopy thinning unevenly. A tree making a calculation about water it doesn't have.
Simon is a Northern Arizona University forestry graduate, an arborist who came home to Arizona in late 2020 to run American Forests' Southwest urban forestry program. He'd trained to read trees. What he walked into was a city where the reading kept changing underneath him. His work now feeds into the Phoenix Metro Urban Forestry Roundtable, a coalition of municipalities, nurseries, and researchers trying to answer a question that won't hold still: what do you plant in a desert that is becoming a different desert?
"It is always hot in the desert, but it has never been this hot for this long."
What reframed his understanding was the nights. Phoenix's 2023 summer brought 31 consecutive days above 110°F, but the real shift was happening after sundown, in multiple stretches when the low never dropped below 90. Nighttime is when saguaros open their pores to release retained water and take in carbon dioxide. When the night stays hot, that exchange fails. Tissues soften. The structural integrity that kept a 40-foot column standing for a century quietly collapses. Three saguaros at the Desert Botanical Garden toppled or lost arms in a single week. "They are not able to release their gasses and toxins at night," Simon explained, "and many rotted from the inside out."
That reading, the moment a saguaro's internal chemistry turns against it, lives in accumulated hours with the plant. No satellite catches it. But the reading depends on a baseline, and Simon was building his baseline in a city where the baseline was already gone.
A framework built for the wrong variable
The USDA Plant Hardiness Zone Map is the standard reference for what to plant where. It measures minimum winter temperatures. It tells you what cold will kill. The 2023 update shifted half the country warmer by a half-zone, reflecting milder winters. The map is silent on what 113 consecutive days above 100°F does to a 20-year-old ficus, or what record nighttime lows do to the biological processes Simon watches for in the field. Phoenix's trees are dying from heat that never relents, measured against a framework calibrated to a question the climate has stopped asking.
Simon works inside that gap every day. Every planting decision he advises on runs through a knowledge system calibrated to the wrong variable. The distance between what the tool measures and what the trees actually face lands on the practitioner, absorbed into professional judgment because no official instrument covers it.
What he sees driving south
Simon has described the geography bluntly. "Places like Paradise Valley have a Tree Equity Score of 100, and you almost wouldn't know you are in the desert: it's lush and well maintained and highly irrigated." Drive thirty minutes southwest to south central Phoenix, "and there is a stark contrast. If there are any trees, they are often stunted or nearly dead."
The city's Tree and Shade Master Plan targets 25% canopy coverage, up from roughly 12%, backed by over $60 million in funding and a tall pot nursery where saplings grow in tubes that force deep taproots so they can survive on natural rainfall alone. The Roundtable's work explicitly targets the neighborhoods with the least shade, which means the trees going into the hottest blocks are also the trees facing the most brutal test. The program's own species guidance now carries a quiet warning:
"Native trees are preferred and should be strongly considered as summer temperatures continue to increase and our monsoons decrease."
That last clause does serious work. Monsoons decreasing means the rainfall those deep taproots are reaching for may not arrive. The Roundtable lists "climate adapted species identification" as an ongoing task. Still being built in real time, by practitioners planting trees they hope will survive conditions nobody has tested them against.
March hits 100
On March 18, 2026, Phoenix recorded its earliest-ever 100°F day. The city plants trees between October and April. The planting season is compressing from the spring end just as the program scales up. Summers arriving earlier, burning hotter, lasting longer. The conditions the trees are meant to address keep narrowing the window in which you can plant them.
Across more than 40 monitoring sites in Maricopa County, teams working with the Roundtable are tracking survival rates, irrigation performance, species-specific failures. The data isn't published yet. The trees are already in the ground.
You plant a palo verde because everything you know says palo verde belongs here. You design a taproot nursery because deep roots are the best hedge against surface drought. You build a 17-species list because those are the trees that work. Then the night doesn't cool, and March hits 100, and a saguaro that stood for a century rots from within because the desert stopped being the desert it was. The knowledge is still in your hands. It holds differently now.
- Saguaro census, ongoing: The Desert Botanical Garden found that 10% of re-observed saguaros were in worse condition than the previous year, with no young cohort growing in the city to replace mature specimens lost to heat stress.
- Cooling benefits under stress: A February 2025 study in AGU Advances found that the cooling benefits of urban trees decline as canopy damage and mortality increase from heat stress, meaning the trees most needed in the hottest neighborhoods may deliver less relief precisely when conditions worsen.
- The $5 million ASU initiative: After 466 confirmed heat-related deaths in 2024, ASU launched an urban forestry accelerator using 3D software to model shade placement across Phoenix, Tempe, Mesa, and Guadalupe, pairing planting plans with workforce training to properly maintain the trees going in.
- Unpublished survival data: The Roundtable's species subcommittee has collected long-term data from over 40 sites across Maricopa County covering survival rates and irrigation performance, but the findings have not yet been made public, leaving practitioners to make planting decisions without the field evidence their own coalition is gathering.